It is desirable to be able to prevent the general dispersion into a room, or other enclosed space, of airborne contaminants such as tobacco smoke, aerosolized drugs, and microorganisms that are emitted from a localized source within the room. It is further desirable to be able to better protect an individual or particular equipment at a localized region of a room from exposure to contaminants that exist in the ambient air of the room. Such a system should provide a relatively even and high rate of air cleaning where space cannot readily be allocated to conventionally used purification equipment.
Many situations arise in which contaminants that are aesthetically undesirable or potentially physically harmful must be removed from the ambient air. An example is the release of pathogenic microorganisms by infected patients in waiting rooms, examining rooms, and hospital wards. A specific problem arises in aerosol therapy, for example, in the treatment of HIV-positive patients with aerosolized pentamidine. During treatment, some of the aerosolized pentamidine escapes from the treatment device and disperses into the air. Further, additional pentamidine can be expelled into the air on droplet nuclei as a result of coughing that is induced in the patient by the treatment process. The attending medical staff have to be protected from the aerosol since chronic exposure to pentamidine reportedly has adverse health effects. The problem is compounded in cases where the patient has an infectious disease such as tuberculosis.
Tobacco smoking is prohibited in many public places to protect against the potentially adverse health effects of "passive smoking." The provision of designated areas for smoking is uneconomic, non-productive, and exasperates employees previously accustomed to smoking in their own offices. A means for preventing the release of tobacco smoke from the immediate vicinity of the smoker would alleviate these problems.
Carpentry is another activity in which a pollutant is emitted from a localized source. Indoor home workshops generate voluminous quantities of sawdust, for example, that are a considerable nuisance and are potentially harmful.
In the examples given above, the pollutant is generated in a localized area, and it is desired to prevent it from dispersing into the surrounding air. A reverse situation can also exist where a person or piece of equipment must be protected from pollutants that may generally exist in the ambient air and to which such person or equipment should not be exposed. Allergy sufferers, for example, are sensitive to a variety of naturally occurring substances such as pollen, spores, dusts, and animal fur. Such an individual might find relief from symptoms by being in a microenvironment rendered substantially free of allergens by a suitably designed air purification system. A localized working environment that is free of pollutants such as dust is also required, for example, for retouching photographic materials, painting small articles, surgical procedures, and other hobby and professional activities.
Indoor air purification has been achieved up to now by a number of different systems. These systems typically contain a fan or blower that circulates the air through a purification means which can generally be referred to here as a filter. The type of filter is selected in accordance with the contaminant that is to be removed. Several types of filters may be used in combination in a single air purifying unit.
The efficacy of a unit in purifying the air in a room is determined primarily by three factors: (1) the effectiveness of the filters in capturing and retaining the pollutant, (2) the rate at which contaminated air is brought to the filters, and (3) the size of recirculating and quiescent regions in the rooms which are not under the influence of the purification system.
The effectiveness of the filters can henceforth be referred to as the filter efficiency. The rate at which the system purifies air is conveniently measured in terms of the number of room volumes that are treated in a given time, and can be expressed as "air ventilations per hour." A system rated at 10 air ventilations per hour, for example, would treat a volume of air each hour which is equal to 10 times the volume of the room in which it is placed. High rates of air ventilation result in rapid removal of contaminants but normally require large blowers and/or filters.
The size of recirculating and quiescent regions, herein referred to as unventilated zones, is affected by the geometry of the room, the furnishings present in the room, and the location and orientation of the inlet and outlet of the air purification system. Special facilities known as "clean rooms" are designed to reduce the size of the unventilated zones. In typical medical, commercial and residential rooms complete elimination of unventilated zones is generally not feasible.
Particles such as dust, pollen and tobacco smoke are removed from the air by particulate filters, many of which function by a mechanical straining process. Filters with very fine straining elements are used to remove particles as small as tobacco smoke and microorganisms. HEPA (High Efficiency Particulate Air) filters are widely used for fine dusts and are rated at efficiencies ranging from 95% to greater than 99.99% for the capture of particles having an average size of 0.3 micrometers, for example. To prevent premature clogging, a coarse filter or "prefilter" may be used to remove larger particles from the air upstream of the HEPA filter.
Mechanical filters designed to have a high efficiency are characterized by their high resistance to air flow, and as a consequence require large and powerful blowers to achieve acceptable air ventilation rates.
Electrostatic filters may also be used to capture particulate pollutants. These function by placing an electric charge of one polarity on the particles which are then attracted to and retained by plates held at the opposite polarity. The advantage of electrostatic filters is that they offer little resistance to the air flow and so can be used in conjunction with small blowers. A disadvantage is that they generate ozone which is itself considered a pollutant.
Gaseous pollutants such as organic vapors, odoriferous contaminants, and radon may be removed by passing the air through an adsorption type filter. Activated carbon is a commonly used adsorbent that captures a wide range of gaseous pollutants. Other adsorbents such as activated alumina and zeolites may be used for the removal of specific contaminants.
The simplest means of indoor pollution control is to vent the contaminated air to the outside without purification. However, the cost of heating or cooling the large volume of replacement air makes this approach uneconomic. In addition, the practice of discharging pollutants without treatment may not be acceptable, particularly in the case of potentially harmful substances.
Utilizing a central ventilating (HVAC) system for air purification is not satisfactory as these systems do not usually provide more than a few air ventilations per hour, whereas guidelines for certain medical environments recommend up to 20 ventilations per hour. Upgrading an HVAC system to achieve such a high ventilation rate is expensive. Restricting the high flows required to specific rooms only within a building might be less costly, but would require special ducts and booster fans.
Another problem with using an HVAC system for such purpose is that it is customary to recirculate some portion of the ventilation air in the building so as to reduce the amount of air drawn in from outside and the associated costs of heating and air conditioning. However, a contaminant released in one room can consequently be spread throughout the building unless all the recirculating HVAC air is purified. Treating the entire HVAC air flow through high efficiency filters imposes an unacceptably high resistance on the blowers. In addition, poorly located inlet and exhaust registers may result in pollutants being dispersed into other rooms or hallways before being drawn into the return air ducts.
The use of special purpose air-purification systems in a room overcomes many of the shortcomings associated with the use of the HVAC system. Available portable air purification units occupy little space, but are generally too small to achieve acceptably high ventilation rates. Space limitations may preclude the use of larger units unless they are mounted in the ceiling area in which case they suffer from the disadvantage of not being portable. Moreover, the efficacy of these units may be adversely affected by the presence of unventilated zones in the room.
One means of overcoming the problem of unventilated zones is the use of a physical enclosure or booth. The enclosure is made large enough to accommodate a person so that the smoker or patient can be seated inside. The pollutant source is thus contained, and by filtering all the air leaving the booth pollutants can be prevented from dispersing into the ambient air. Physical enclosures, however, suffer from the disadvantage of occupying even more space than conventional air cleaning systems, and they are not readily portable. In addition, the need to place a person in a confined space is intimidating in the case of a patient, and impractical in the case of an office worker, for example.